PCB Tech - PCB process analysis of basic problems of connector terminal gold plating

PCB process analysis of basic problems of connector terminal gold plating

Connector gold plating layer common quality problem analysis Connector gold plating layer common quality problem analysis

1 Introduction
In connector electroplating, due to the higher electrical performance requirements of the contact pair, the gold plating process occupies an obvious important position in the connector electroplating. At present, except for some strip connectors adopting selective electroplating gold process, the remaining large number of needles The gold plating in the hole of the hole parts is still carried out by barrel plating and vibration plating. In recent years, the volume of the connector has become more and more miniaturized, and the quality of the gold plating in the hole of the pinhole parts has become increasingly prominent. The quality requirements of the connector are getting higher and higher, and some users are even very picky about the appearance quality of the gold layer. In order to ensure the quality and bonding force of the connector gold layer, these types of common quality problems are always the key to improving the gold plating quality of the connector. Let's discuss the reasons for these quality problems one by one.

2 Reasons for the quality of the gold-plated layer
2.1 The color of the gold layer is abnormal
The color of the gold-plated layer of the connector is inconsistent with the color of the normal gold layer, or the color of the gold layer of different parts in the same supporting product is different. The reasons for this problem are:
2.1.1 The impact of gold-plated raw materials impurities
When the impurities introduced by the chemical materials added to the plating solution exceed the tolerance of the gold plating solution, it will quickly affect the color and brightness of the gold layer. If it is affected by organic impurities, the gold layer will become dark and bloom. The film inspection is dark and the location of the flower is not fixed. If the interference of metal impurities will cause the effective range of the current density to be narrowed, the Haul trough test shows that the current density of the test piece is not bright at the low end or the high end is not bright and the low end is not plated. Reflected on the plated parts, the plating layer is reddish or even black, and the color changes in the holes are more obvious.

2.1.1 Gold plating current density is too large
Due to the calculation error of the total area of the plating tank parts, the value is larger than the actual surface area, so that the gold plating current is too large, or the amplitude is too small when vibrating gold electroplating, so that all or part of the plating gold plating layer in the tank is rough, and the gold is visually observed. Layers are red.

2.1.3 Aging of the gold plating solution
If the gold plating solution is used for too long, the excessive accumulation of impurities in the plating solution will inevitably cause the color of the gold layer to be abnormal.
2.1.4 The alloy content in the hard gold coating changes
In order to improve the hardness and wear resistance of the connector, the connector gold plating generally adopts the hard gold plating process. Among them, gold-cobalt alloy and gold-nickel alloy are used more. When the content of cobalt and nickel in the plating solution changes, it will cause The color of the gold plating layer changes. If the cobalt content in the plating solution is too high, the color of the gold layer will be reddish; if the nickel content in the plating solution is too high, the metal color will become lighter; if the change in the plating solution is too large, the same supporting product is different When the parts are not gold-plated in the same tank, the color of the gold layer of the same batch of products provided to the user will be different.
2.2 The hole cannot be plated with gold
When the thickness of the outer surface of the plated part reaches or exceeds the specified thickness value after the gold plating process of the plug or socket of the connector is completed, the inner hole plating of the wire hole or socket is very thin or even has no gold layer.
2.2.1 The plated parts are inserted into each other when gilding
In order to ensure that the jack of the connector has a certain degree of flexibility when the jack is plugged and used, most types of jacks are designed with a split groove at the mouth during product design. During the electroplating process, the plating parts are constantly turning over. The jacks are inserted into each other at the opening, so that the power lines of the plug-in parts are shielded from each other, and the plating in the hole is difficult.
2.2.2 Plated parts are connected end to end when gilding
For some types of connectors, the outer diameter of the pin bar is slightly smaller than the aperture size of the wire hole during the design of the pin. During the electroplating process, some of the pins will form end to end, causing the wire hole to not be plated with gold. The above two phenomena are more likely to occur during vibration gilding.
2.2.3 The concentration of blind holes is larger than the thickening ability of the electroplating process
Since there is still a distance between the bottom of the split groove of the jack and the bottom of the hole, this distance objectively forms a blind hole. There is also such a blind hole in the wire hole of the pin and the jack, which is to provide wire welding When the hole diameter is small (often less than 1 mm or even less than 0.5 mm) and the concentration of the blind hole exceeds the hole diameter, it is difficult for the plating solution to flow into the hole, and it is difficult for the plating solution to flow into the hole. It flows out, so the quality of the gold layer in the hole is difficult to guarantee.
2.2.4 The area of the gold-plated anode is too small
When the size of the connector is small, the total surface area of the single-slot plated parts is relatively large, so if there are more single-slot plated parts when plating small pinhole parts, the original anode area is not enough. Especially when the platinum titanium mesh When the platinum is used for too long and there is too much loss, the effective area of the anode will be reduced, which will affect the deep plating ability of the gold plating, and the holes of the plated parts will not be plated.
2.3 Poor adhesion of the coating
When inspecting the bonding force of the coating of the connector after plating, sometimes the front part of the pin end of the pin is bent or the wire hole of the pinhole part is flattened when the coating is peeled, sometimes at high temperatures. (2001 hours) The detection test found that the gold layer had very small blisters.
2.3.1 Incomplete treatment before plating
For small pinhole parts, if trichloroethylene ultrasonic degreasing cannot be used immediately after the machining process is completed, it is difficult to remove the dried oil in the hole by the following conventional plating pretreatment, so that the plating in the hole The binding force will be greatly reduced.
2.3.2 Incomplete activation of the substrate before plating
Various types of copper alloys are widely used in the connector matrix materials. The iron, lead, tin, beryllium and other trace metals in these copper alloys are difficult to activate in the general activation solution. If they are not activated by the corresponding acid, electroplating is carried out. At this time, the oxides of these metals and the coating are difficult to combine, so the phenomenon of high temperature blistering of the coating is caused.
2.2.3 The concentration of the plating solution is low
When using sulfamate nickel plating solution for nickel plating, when the nickel content is lower than the process range, the quality of the plating layer in the hole of the small pinhole parts will be affected. If the gold content of the pre-plating solution is too low, then the hole during gold plating There may be no gold plated inside.When the plated part enters the thick gold plating solution, the nickel layer in the plated hole of the hardware layer in the hole has been passivated.As a result, the bonding force of the gold layer in the hole is naturally poor.
2.3.4 The current density of slender pins is not reduced during electroplating
When plating a thin and long-shaped pin, if the plating is carried out according to the usual remote current density, the plating layer on the needle tip will be much thicker than that on the needle shaft, and the tip of the needle may sometimes be smaller than the shape of the match head when observed under a magnifying glass. (see Figure 3) The coating on the neck, that is, the gold coating on the top of the pin front end, is not qualified in the bonding test. This phenomenon is easy to occur during vibration gold plating.
2.3.5 Vibration gold-plated vibration frequency adjustment is incorrect
When using vibration electroplating plating connectors, if the vibration frequency is not adjusted correctly during nickel plating, the plated parts will jump too fast, and the easy-to-open double-layer nickel will have a great influence on the bonding force of the plating.
3 ways to solve quality problems
3.1 Eliminate factors that affect electroplating quality from product design
First of all, when the connector is designed for the product, it is necessary to consider the possible impact on the electroplating process, and try to avoid the hidden dangers of the electroplating quality due to improper design considerations.
3.1.1 For the socket with a straight opening, when splitting the slot, the opening is inclined at a 45-degree angle from the edge of the mouth to the center of the mouth, and then vertically downwards along the center of the mouth. If it is a cross-shaped opening, Hole, you can close the socket first to make the chain width of the split notch smaller than the thickness of the socket wall, so as to reduce and avoid the plugging phenomenon of the sockets during gold plating (see Figure 4).
3.1.2 When designing, the size of the pin bar of the pin should always be slightly larger than the size of the wire hole hole or extend the length of the milling arc of the wire hole to avoid connecting the pins end to end during electroplating.
3.1.3 A horizontal through hole is designed at the bottom of the blind hole to allow the plating solution to enter and exit the hole smoothly during electroplating.
3.2 Adopt scientific electroplating process management methods
3.2.1 Strengthen the quality control of electroplating, especially the quality of the gold salt. For each batch of gold salt used, in addition to the routine physical and chemical inspections, samples must be sampled for the Haul tank test, and the test is confirmed to be qualified. Used in plating tanks. Haul tank test method: Take 12 grams of gold salt in the sample, add 100 grams of potassium citrate to prepare 1 liter of plating solution, heat to 50 degree Celsius to adjust the pH 5.4-5.8 for the Haul tank test. The normal result is 250 ml The Haul tank test sample should be electroplated with 0.SA current for 1 minute. The light range should be more than one-half of the area at the low end of the current density.
3.2.2 Calculate and record the quantity, surface area, and total current of the plated parts in each tank before electroplating, so as to find the cause after the quality problem occurs.
3.2.3 Analyze and adjust the plating solution in time according to the gold plating production situation to ensure that the composition of the plating solution is within the optimal process range. The nickel plating solution should be treated with activated carbon at least every month. When the gold plating solution is used for more than 70 cycles, it should be considered to prepare a new plating solution, and the old plating solution should be recycled after gold is discarded.
3.2.4 Ensure that there is enough anode area during gold plating. When there are a lot of bubbles on the platinum-titanium mesh used in the electroplating process and the gold-plating can not be plated for a long time, consider replacing with a new platinum-titanium anode.
3.2.5 Add an ultrasonic degreasing cleaning process for small pinhole parts before plating.
3.2.6 Boil the beryllium bronze connector with 1:1 hydrochloric acid solution for 10-30 minutes before plating, and then enter the conventional electroplating process after fully removing oxides. For brass parts, add it to the activation solution before nickel plating A certain amount of hydrofluoric acid or directly use fluoride with fluoride to prepare an activation solution to ensure the activation of trace metals in the base copper alloy.

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